Collets for use in tool-holding chuck assemblies are well known. Such collets generally comprise a tubular body formed from a plurality of elongated, flexible steel fingers. The fingers are separated by longitudinal collet saw slots that impart some degree of radial flexibility to the collet, which allows the collet to selectively grip the shank of a cutting tool, such as a drill bit. Adjacent gripping fingers are interconnected by an alternating pattern of metal webs to form a one-piece collet body. In operation, the collet body is inserted in a complementary-shaped opening in a chuck shaft so that a distal end of the collet body projects out of the shaft. An annular lock nut having an inner diameter screw thread that matches an outer diameter screw thread on the shaft is then installed over the shaft and distal end of the collet body. The lock nut has a nose ring with a frustoconical cam surface that engages the distal end of the collet body and squeezes it radially inwardly as the lock nut is screwed onto the chuck shaft. The radial compression that the lock nut applies to the distal end of the collet body flexes the body inwardly, creating a gripping force between the inner diameter of the collet body and a tool shank inserted therein.
Unfortunately, existing lock nuts used for tightening collets into the chuck have a tendency to disengage with the collet if the operator is not careful when installing the collet into the lock nut. If the collet is accidentally disengaged from the lock nut prior to assembly with the chuck, the collet will lock down properly, but the lock nut will not be able to remove the collet from the chuck. The lock nut can be unscrewed from the chuck, but the collet and cutting tool are left inside the chuck. When this occurs, the only way to remove the collet is to unthread the retention knob and drive the collet out from the rear of the tool holder by using a drift.
One solution to this problem is to clip the collet inside the lock nut prior to tightening the lock nut onto the tool holder chuck. This method uses an eccentric oval arranged inside the lock nut that is machined either larger or slightly smaller than the collet diameter. One example of such an eccentric oval arrangement is described in WO00/14502. Although the collet and lock nut will be held together to prevent misalignment during assembly, this method provides only two points of contact on the lock nut to draw the locked collet and cutting tool out of the tool holder chuck. In addition, additional machining must take place to balance the eccentric bore.
Thus, there is a need for an improved collet and lock nut assembly that is naturally balanced, while allowing easy entry of the collet into the lock nut, while providing additional points of contact to assure that the lock nut will remove the collet and cutting tool from the chuck.